The Mingler process periodically allocates most of the RAM I have on my MacBook Pro, bringing it to a halt. I have never just let it run, though I am doing that now just to see what will happen. I've read through discussions at support.apple.com and none of those helped. How can I control Mingler? More generally, is there a way to limit processes using a system resource on Mac OS X Lion?
I think Mingler is a process related to iCal or the Sync Utility. see here: https://discussions.apple.com/thread/2178706?threadID=2178706 for more details as of how to make it go aways.
Mingler is responsible for doing a lot of the work of the deprecated Sync Services framework -- iCloud contacts, bookmarks, calendar entries, etc. that are divergent between your Mac and the cloud are sorted out by this process.
There can be a lot of different reasons why it may go haywire; corrupt data of some sort is usually a good lead. You can try disabling the different sync options in the iCloud preference pane and see which, if any, improves matters. (See TidBITS here http://tidbits.com/article/12796 for an example of corrupt bookmark sync.)
You can also try disabling legacy Sync Services in Outlook (if you use it) and other forms of contact or calendar sync and then reset the Sync Services database, via Apple's instructions here: OS X: Resetting Sync Services (Internet archive)
If your machine is still covered by AppleCare (or even if it's not), I encourage you to report these issues on Apple's support site or via a Genius Bar appointment. This problem is prevalent enough that it deserves more attention from Apple.